Project Summary/Abstract The long-term objective of this proposal is to understand why bariatric surgery is such an effective treatment for obesity and its associated metabolic complications. Peripheral metabolic signals communicate levels of energy stores to the brain and elicit a host of neuronal responses that maintain energy balance; such regulatory mechanisms make it difficult to maintain diet-induced weight loss. The goal is to understand how these central regulatory mechanisms are circumvented following surgical alterations in the gut. Human studies have been limited by the lack of rigorous diet-induced weight loss controls and lack of biochemical measurements reflecting central brain pathways. This proposal will focus on key brain pathways hypothesized to mediate the effects of surgery on energy balance as well on identifying new pathways in subjects after Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (SG) compared to carefully matched dietinduced weight loss controls. The innovation is in the use of cerebrospinal fluid (CSF) neuropeptide, hormone and protein measurements as a surrogate for changes in brain activity. In addition to studying neuropeptides (melanocortin, opioid), hormones (leptin and gut hormones) and neurotransmitters (dopamine and serotonin) that have been implicated in this process, proteomic analysis will be used to uncover new biomarkers that are unique to surgical weight loss. Our preliminary proteomic data have identified a pattern of changes in CSF that occurs after diet-induced weight loss and forms the basis for determining how this pattern is altered after surgery. An important focus will be on the melanocortin system consisting of the proopiomelanocortin (POMC)derived MSH peptides and the MSH antagonist, agouti related protein (AgRP), that plays a critical role in regulating energy balance and in responding to weight loss and is impacted by leptin and gut hormones. CSF levels of the POMC prohormone can serve as a marker of central POMC activity and we have shown striking correlations of CSF POMC with BMI and leptin. CSF POMC and AgRP levels decrease and increase respectively following diet-induced weight loss; plasma AgRP also increases. Given the effects of RYGB and SG on gut hormone levels, we will measure ghrelin, GLP- 1, PYY and FGF19, as they can all affect melanocortin activity and metabolism. CSF leptin and soluble leptin receptor will also be measured to assess effects on leptin transport into brain. Another focus will be on the HPA axis which has bidirectional interactions with the brain melanocortin system. Our data show that CSF cortisol increases after weight loss. Our hypothesis is that distinct biochemical changes will occur in CSF after diet- induced weight loss and this pattern will be altered after RYBG and SG; CSF proteomic analysis will validate changes expected to occur in known pathways as well as identify new pathways responsible for the dramatic effects of bariatric surgery. Understanding the mechanisms through which surgery produces long-term weight loss is highly significant and paramount to developing new drug targets and filling the therapeutic void in the treatment of obesity.